1. Field of the Invention
The present invention is directed to a method and apparatus for measuring the size of the capsular bag of a human eye.
2. Description of the Related Art
Recently, a great deal of effort has been devoted to developing an accommodating intraocular lens, which can adjust its power over a particular range to clearly view both near and far objects. The accommodating intraocular lens is generally inserted into the capsular bag of the eye, which is a transparent structure that houses the natural lens of the eye and generally remains in the eye after the natural lens has been surgically removed.
The accommodating intraocular lens changes its power in response to a squeezing and/or expanding force applied to the lens by the capsular bag via the ciliary muscle.
It is generally important to know the size (or more precisely, the inner diameter or circumference) of the capsular bag for each patient's eye, prior to insertion of the intraocular lens. The capsular bag size may vary patient-to-patient or eye-to-eye, and if the bag is larger or smaller than expected, the lens may end up slightly expanded or squeezed upon implantation. This, in turn, may result in a shift in the nominal base power and/or a reduction in the accommodation range, both of which are undesirable.
Although the capsular bag diameter is a desirable and useful quantity, it is also quite difficult to measure.
There have been attempts to measure the capsular bag size with ultrasound. While ultrasound may be useful for determining the central thickness of the unoperated lens, it is not generally versatile enough to image the entire crystalline lens, and cannot reliably read out to the perimeter of the lens.
There have been attempts to measure the capsular bag by inserting a capsular tension ring (CTR) into the eye. See, for instance, K. STRENN, R. MENAPACE, and C. VASS, “Capsular bag shrinkage after implantation of an open-loop silicone lens and a poly(methyl methacrylate) capsule tension ring,” J Cataract Refract Surg, 1997, pp. 1543-1547, Vol. 23, which is hereby incorporated by reference in its entirety. In this reference, a CTR indicates the capsular diameter, based on linear measurement of a peripheral gap. After the measurement, the CTR is generally not removed from the eye and remains resident in the eye, which may be undesirable.
There have been attempts to correlate capsular bag size with other eye properties that can be measured more easily. See, for instance, C. VASS, R. MENAPACE, K. SCHMETTERER, O. FINDL, G. RAINER AND I. STEINECK, “Prediction of pseudophakic capsular bag diameter based on biometric variables,” J Cataract Refract Surg, October 1999, pp. 1376-1381, Vol. 25, which is hereby incorporated by reference in its entirety. In this reference, measurements of capsular bag diameter were taken on a sample of patients, using the CTR noted above. In addition, measurements of corneal power and axial length were taken on the same patients, using known methods. A regression analysis of the measurements produced a statistically significant correlation between capsular bag diameter and corneal power and axial length, but not with a sufficient accuracy for predicting the required size of an accommodating intraocular lens.
There have been attempts to convert the capsular bag circumference dimension to a linear dimension, then to measure the linear dimension with a camera or visually. See, for instance, M. TEHRANI, H. B. DICK, F. KRUMMENAUER, G. PFIRRMANN, T. BOYLE and B. STOFFELNS, “Capsule measuring ring to predict capsular bag diameter and follow its course after foldable intraocular lens implantation,” J Cataract Refract Surg, November 2003, pp. 2127-2134, Vol. 29, which is hereby incorporated by reference in its entirety. In this reference, a Koch capsule measuring ring is inserted into the eye. The ring is an incomplete circle, with appendices on each end, so that when the ring is inserted into the capsular bag, the separation between the appendices is related to the capsular bag circumference. The ring is left in the eye after the measurement is taken, which may be undesirable.
In addition, for the above reference, the measurement of the appendix separation may be disadvantageous for two reasons. First, the measurement is taken at the peripheral edge of the eye, which is a difficult region of the eye for measurement. For instance, the region to be measured might be outside the area of the pupil, and might require use of a slitlamp, or unusual and undesirable handling of the pupil. Second, it is difficult to measure a linear dimension in the eye. Often, such a measurement is taken through the cornea, which can magnify the linear dimension, especially at the periphery of the eye. Because corneal powers may vary from patient-to-patient and eye-to-eye, there may be a significant uncertainty in any linear measurements taken through the cornea. In addition, because most eye surgery is performed through a microscope, the measurement may have to be taken through the microscope, which may have a zoom feature or a variable focal length that may further complicate a linear dimension measurement.
Accordingly, there exists a need for an apparatus and method for measuring the size of the capsular bag of an eye, which does not rely on a linear measurement at the periphery of the eye.